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LIBRARY OF GALAXY HISTORIES RECONSTRUCTED FROM MOTIONS OF STARS

** Synopsis: The CALIFA survey allows to map the orbits of the stars of a sample of 300 galaxies, a fundamental information to know how they formed and evolved. **

Just like the Sun is moving in our galaxy, the Milky Way, all the stars in galaxies are moving, but with very different orbits: some of the stars have strong rotations, while others may be moving randomly with no clear rotation. Comparing the fraction of stars on different orbits we can find out how galaxies form and evolve. An international team of astronomers has derived directly, for the first time, the orbital distribution of a galaxy sample, containing more than 300 galaxies of the local universe. The results, published in Nature Astronomy, are based on the CALIFA survey, a project developed at Calar Alto Observatory and conceived from the Institute of Astrophysics of Andalusia (IAA-CSIC).

Galaxies are the largest structures in the universe, and scientist study how they evolve to understand the history of the universe. Galaxy formation entails the hierarchical assembly of halos of dark matter (a type of matter that has not been directly observed and whose existence and properties are inferred from its gravitational effects), along with the condensation of normal matter at the halos’ center, where stellar formation takes place. Stars that formed from a settled, thin gas disk and then lived though dynamically quiescent times will present near circular orbits, while stars with random motions are the result of turbulent environments, either at birth or later, with galactic mergers.

Thus, the motions of stars in a galaxy are like a history book; they record the information about their birth and growth environment, and it may tell us how the galaxy was formed. “However, the motion of each single star is not directly observable in external galaxies. External galaxies are projected on the observational plane as an image and we cannot resolve the discrete stars in it,” says Ling Zhu, researcher from the Max Planck Institute for Astronomy who leads the study. “The CALIFA survey uses a recently developed technique, integral field spectroscopy, which can observe the external galaxies in such a way that it provides the overall motion of stars. Thus, we can get kinematic maps of each galaxy.”

The researchers then build models for each galaxy by superposing stars on different types of orbits. By constraining the model with the observed image and kinematic maps, they can find out the amount of stars moving on different types of orbits in each galaxy. They call it the stellar orbit distribution and, for this study, the team has built models for all 300 galaxies, representative of the general properties of galaxies in the local universe.

The maps show changes in galactic orbit distribution depending on the total stellar mass of the galaxies. The ordered-rotating orbits are most prominent in galaxies with total stellar masses of 10 billion solar masses, and least important for the most massive ones. Random-motion orbits unsurprisingly dominate the most massive galaxies (more than 100 billion solar masses). “This is the first orbit-based mass sequence across all morphological types. It includes flourishing information of a galaxy’s past, basically whether it had been a quiet succession of only smaller mergers or shaped by a violent major merger. Further studies are needed to understand the details,” says Glenn van de Ven (ESO).

The researchers had found a new and accurate method of reading off a galaxy’s history – and their survey with its data sets for 300 galaxies turned out to be the largest existing library of galaxy history books.

“This work highlights the importance of integral field spectroscopy and, in particular, of large-scale surveys such as the CALIFA project. The significant contribution of what we call ‘hot’ orbits, a mixture of rotation and random movements of the stellar component, poses important challenges to cosmological models of galaxy formation and evolution,” says Rubén García Benito, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) participating in the project.

CALIFA’s results represent an observationally-determined orbit distribution of galaxies in the present-day universe. They lend themselves thus to direct comparison with samples of cosmological simulations of galaxies in a cosmological context. In this sense, these results open a new window for comparing galaxy simulations to the observed galaxy population in the present-day universe.

A NEW APPROACH FOR DETECTING PLANETS IN THE ALPHA CENTAURI SYSTEM

Yale astronomers have taken a fresh look at the nearby Alpha Centauri star system and found new ways to narrow the search for habitable planets there.

According to a study led by Professor Debra Fischer and graduate student Lily Zhao, there may be small, Earth-like planets in Alpha Centauri that have been overlooked. Meanwhile, the study ruled out the existence of a number of larger planets in the system that had popped up in previous models.

“The universe has told us the most common types of planets are small planets, and our study shows these are exactly the ones that are most likely to be orbiting Alpha Centauri A and B,” said Fischer, a leading expert on exoplanets who has devoted decades of research to the search for an Earth analog.

The new study appears in the Astronomical Journal. Co-authors are John Brewer and Matt Giguere of Yale and Bárbara Rojas-Ayala of Universidad Andrés Bello in Chile.

The Alpha Centauri system is located 1.3 parsecs (24.9 trillion miles) from Earth, making it our closest neighboring system. It has three stars: Centauri A, Centauri B, and Proxima Centauri. Last year, the discovery of an Earth-like planet orbiting Proxima Centauri set off a new wave of scientific and public interest in the system.

“Because Alpha Centauri is so close, it is our first stop outside our solar system,” Fischer said. “There’s almost certain to be small, rocky planets around Alpha Centauri A and B.”

The findings are based on data coming in from a new wave of more advanced spectrographic instruments at observatories located in Chile: CHIRON, a spectrograph built by Fischer’s team; HARPS, built by a team from Geneva; and UVES, part of the Very Large Telescope Array. “The precision of our instruments hasn’t been good enough, until now,” Fischer said.

The researchers set up a grid system for the Alpha Centauri system and asked, based on the spectrographic analysis, “If there was a small, rocky planet in the habitable zone, would we have been able to detect it?” Often, the answer came back: “No.”

Zhao, the study’s first author, determined that for Alpha Centauri A, there might still be orbiting planets that are smaller than 50 Earth masses. For Alpha Centauri B there might be orbiting planets than are smaller than 8 Earth masses; for Proxima Centauri, there might be orbiting planets that are less than one-half of Earth’s mass.

In addition, the study eliminated the possibility of a number of larger planets. Zhao said this takes away the possibility of Jupiter-sized planets causing asteroids that might hit or change the orbits of smaller, Earth-like planets.

“This is a very green study in that it recycles existing data to draw new conclusions,” said Zhao. “By using the data in a different way, we are able to rule out large planets that could endanger small, habitable worlds and narrow down the search area for future investigations.”

This new information will help astronomers prioritize their efforts to detect additional planets in the system, the researchers said. Likewise, the continuing effort by Fischer and others to improve spectrographic technology will help identify and understand the composition of exoplanets.

Illustration by Michael S. Helfenbein

Taurus constellation

Astronaut John W. Young, commander of the Apollo 16 lunar landing mission, works at the Lunar Roving Vehicle (LRV) just prior to deployment of the Apollo Lunar Surface Experiments Package (ALSEP) during the first extravehicular activity (EVA-1) on April 21, 1972. [3072 x 3072]